clean: change to bsd2 license

Clean is released under the bsd2 license as can be seen here:
https://clean.cs.ru.nl/Download_Clean

For the full license, see:
https://ftp.cs.ru.nl/Clean/Clean30/CleanLicenseConditions.txt

pkgs/development/compilers/clean/default.nix

@@ -1,7 +1,8 @@
 { lib, stdenv, fetchurl }:
 
 stdenv.mkDerivation {
-  name = "clean-3.0";
+  pname = "clean";
+  version = "3.0";
 
   src =
     if stdenv.hostPlatform.system == "i686-linux" then (fetchurl {
@@ -45,8 +46,8 @@ stdenv.mkDerivation {
     '';
 
     homepage = "http://wiki.clean.cs.ru.nl/Clean";
-    license = lib.licenses.lgpl21;
-    maintainers = [ lib.maintainers.kkallio ];
+    license = lib.licenses.bsd2;
+    maintainers = [ lib.maintainers.erin ];
     platforms = [ "i686-linux" "x86_64-linux" ];
   };
 }